Thus on the first day of the battle, Whitsunday 21 May, Napoleon was on the island of Lobau, cut off from the south bank of the Danube with about 24,000 men and 60 cannon facing the full might of the Austrian Hauptarmee (circa 95,000 men with 200 cannon). Masséna's 4th corps occupied a position around the larger village of Aspern facing the Austrian right, Molitor's division lodged in Aspern itself, Lasalle, Espagne, Nansouty from Bessières' Cavalry division were set in the centre and Boudet with St-Hilaire and Lannes' 2nd corps was ranged in and around the smaller village of Essling. As for the Austrian position on that morning, the main army was stationed slightly to the west of Wagram. They received orders early in the morning of 21 to make an attack in five columns. The first, second and third columns were to march down the north bank of the Danube on Aspern. The fourth column was to head for Essling, and the fifth east of Essling for Gross-Enzersdorff. Plans were also laid to sabotage the French bridges by launching boats, floating mills and other debris upstream.

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Napoleon's position was however relatively weak. He had few troops, no infantry but only cavalry to cover the central part of his line between Aspern and Essling, and although large numbers of troops were milling on the southern bank of the Danube waiting to cross over to Lobau the bridges were down and repairs were slow and difficult. To compound this problem, Napoleon was not entirely sure where the main body of the Austrian army stood. His decision to offer battle on the Sunday morning was somewhat bold. Indeed his initial thoughts were to refuse the fight. However the sound of cannon at Aspern and the news that the bridges had been repaired convinced him that he should stand his ground.3 And so on the Sunday afternoon, Hiller launched a forceful attack against Masséna at Aspern. The marshal held in a desperate struggle against the slightly disordered Austrian attack. The Duc de Rivoli performed miracles, appearing everywhere with his gun in hand, guiding and encouraging his men. When his generals complained that their conscripts were terrified by the carnage, he bellowed at them (with his heavy Niçois accent): “get them drunk and show them the flag!” As for Lannes, he was keeping the Austrians at bay at Essling, whilst Bessières' cavalry held the zone between the two villages and attracting a great deal of cannon fire from all sides – the fields were said to be zigzagged by trails of cannon fire from all sides. By nightfall the French forces had not crumbled and with the bridges once again repaired the number of troops and cannon rose to 31,500 and 90 respectively.

The second day was to be different story. The rest of the 2nd corps of the French army, under Général de Division Oudinot, the Guard and its cavalry crossed the bridges at 3am thus doubling Napoleon's available forces. A dawn attack brought Aspern once again under French control and an Austrian attempt to seize Essling was halted at 5am. Despite the fact that the French army was still not up to strength (Davout's 3rd corps still stood on the southern bank of the Danube), Napoleon decided launch an attack on Austrian centre. Oudinot's two divisions led by Lannes and supported by cavalry would have broken the enemy line had it not been for timely action on the part of the Austrian generalissimus Karl who in the thick of the action lifted the standard and prevented a certain rout. Napoleon was similarly to be seen in the heat of the battle directing cannon and providing an important psychological lift simply by his presence. At this crisis point in the struggle, however, news reached the French emperor that the bridges were once again down, definitively preventing the long-awaited arrival of Davout and his troops. Significantly short of ammunition, Napoleon ordered Lannes to pull back and the Austrians then retook most of Essling and threatened the French right flank. By 1-30pm the French position had become untenable and at 2pm the emperor gave the order to fall back onto the island of Lobau. The retreat, which was facilitated by the Young Guard retaking Essling, went on for the rest of the day, with the bridge linking Lobau to the northern bank of the Danube finally being dismantled at 3-30am on 23 May.

The two-day battle was exceedingly fierce and bloody. French and Austrians losses were huge (21,000 and 23,000 respectively). Both Karl and Napoleon had frequently risked their live in the thick of the fighting. Artillery played a significant role, being the source of the most of the casualties, most notably amongst the officers on both sides. The Austrian army lost thirteen generals, both Liechtenstein and Hohenzollern receiving injuries. The French army on the other hand suffered the irreplaceable loss of Maréchal Lannes not to mention that of the exceedingly able generals Espagne and St-Hilaire. As Jacques Garnier has noted, the French failure was due in large part to poor evaluation of the risk of crossing the Danube and insufficient reconnaissance before the battle. However it was a field of glory for Masséna who was later to be decorated Prince d'Essling for the extraordinary feats he performed that day.

For the Austrians it was the Achtungserfolg or symbolic victory which the archduke had sought. An army commanded by Napoleon in person had been driven back. The French emperor's aura of invincibility had been significantly tarnished, and unlike Eylau the whole of Europe was soon to learn of it. But the French army was not significantly weakened. And the setback only made Napoleon all the more determined to get it right at the forthcoming battle which would decide the campaign…